To take benefit of the best prices you will need to prove you are a leisure traveller, on the way to heathrow airport, so you will need to give either yr flight numbers or your holiday booking reference. I don't think you will need to give this when you arrive at northampton station but you may be asked to provide them on your journey from northampton station to the airport.
Going to heathrow airport by train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these inexpensive tickets from the station to the airport is that there must be at least two adults travelling together. But another great advantage is that kids go free. All children under 5 go free, and then one kidover five but under sixteen per adult travelling by train to the airport.
make sure you leave plenty of time to get to the airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you have spare time at heathrow airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to worry about the opening hours of northampton station.
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